Currently, memory technology is being continuously developed with the advancement of microelectronics industry. An object of the memory industry is to enhance integration density and reduce manufacturing cost. A non-volatile memory has an advantageous ability of holding data without power supply and thus plays an important role in information storage field.
There has been developed a new-type non-volatile memory using resistive-switching material(s). The non-volatile memory has advantages such as high speed (<5 ns), low power consumption (<1V), high storage density, and adaptability for integration, and thus is a competitive candidate for the next-generation semiconductor memory. The resistive-switching memory typically has an M-I-M (Metal-Insulator-Metal) structure. That is, a resistive-switching material layer is sandwiched between two metal electrodes.
The resistive-switching material is typically a transition metal oxide, such as NiO, TiO2, HfO2, ZrO2, or ZnO, etc. The resistive-switching material can present two stable states: a high-resistance state corresponding to data ‘0’ and a low-resistance state correspond to data ‘1’. Transition from the high-resistance state to the low-resistance state is called a program or set operation, and transition from the low-resistance state to the high-resistance state is called an erase or reset operation.
The resistive-switching memory devices can be categorized into a unipolar type and a bipolar type according to the operation modes thereof. In the operation of a unipolar resistive-switching memory device, voltage levels of a same polarity are applied across the device. The resistive-switching material switches between the high-resistance state and the low-resistance state under control of the amplitude of the applied voltage to implement a write or erase operation of data. In the operation of a bipolar resistive-switching memory device, however, voltage levels of opposite polarities are applied across the device to control the resistive-switching material to switch between the high-resistance state and the low-resistance state. The bipolar resistive-switching memory device has better performances than the unipolar memory device in switching speed, device uniformity, reliability (e.g., ability of maintaining data and switchable number), and controllability.
The resistive-switching memory devices can be categorized into a 1T-1R type and a 1D-1R type according to the basic configurations thereof. In a 1T-1R configuration, each memory cell comprises a gating transistor and a resistive-switching element. Data can be written into or erased from a specific memory cell by controlling the gating transistor. Most area of the memory cell is occupied by the gating transistor, which constitute a severe obstacle in improving the integration of the memory device. In a 1D-1R configuration, each memory cell comprises a diode and a resistive-switching element. Data can be written into or erased from a specific memory cell by controlling the diode. Because the diode has an area smaller than that of the transistor, 1D-1R configuration is more advantageous in enhancing integration.
A Schottky diode based on metal-semiconductor contact principle has a large backward current controlled by the metal material and externally-applied bias voltage. The inventor realized that a 1D-1R resistive-switching random access memory device that operates in the bipolar mode can be achieved by connecting a bipolar resistive-switching element in series with a Schottky diode, which is selected to have suitable diode switch parameters under forward bias and backward bias.
However, the Schottky diode must have a sufficient area to provide the current required in driving the resistive-switching cell to switch between respective resistance states due to the limitation of current density. This prevents a further enhancement of the storage density of the resistive-switching random access memory device.